1. Field of the Invention
The present invention relates to a color printer for recording a color image in photosensitive material and more particularly to a color printer utilizing a micromirror device in which small size mirrors are disposed in matrix.
2. Description of the Related Art
A micromirror device having extremely small-sized mirrors (hereinafter micromirror) is suggested. The micromirrors are arranged in line or in matrix and the tilt angle of each micromirror is controlled to deflect the incident-light. The micromirror devices include digital micromirror devices (DMD) which tilt each micromirror by electrostatic force, piezoelectric drive type micromirror device (AMA) which tilt each micromirror by a fine piezoelectric element and the like.
For example, in a digital micromirror device, each of the micromirrors keeps horizontal state when a power supply is off and is capable of swinging between a valid reflection state (ON-state) and an invalid reflection state (OFF-state) according to one bit mirror drive data value written to a memory cell. In the valid reflection state, the micromirror tilts by +.theta. relative to a vertical line. In the invalid reflection state, the micromirror tilts by -.theta. relative to a vertical line.
The digital micromirror device is obliquely lighted by parallel rays from a light source. While the micromirror is set in the valid reflection state, spot light reflected by the micromirror is guided to an image forming optical path. A projector lens is disposed at the image forming optical path so that spot light is projected on an image formation face of photosensitive recording medium via the projector lens. While the micromirror is set in the invalid reflection state, spot light reflected by the micromirror is guided to an eliminating optical path. The spot light is absorbed by a light absorption member for example.
The digital micromirror device may be utilized for a projector or a printer as a image forming device. About utilization for the projector, it is described in a monthly magazine "O plus E", October, 1994, pp. 90-94. About utilization for the printer of electrophotography type, it is described in "Digital Micromirror Device bar for hardcopy", Vol 2413, 1995, SPIE.
In the projector described in the foregoing document, an area-type digital micromirror device is employed and image of one frame is projected on a screen. The area-type digital micromirror device is provided with N micromirror arrays which are arranged side by side. Each of the micromirror arrays is provided with M micromirrors which are disposed in line.
For medium contrast image, amount of the spot light projected on the screen is controlled by means of pulse width modulation (PWM). The highest bit of K-bit image data is picked up and written to the micromirror device as a mirror drive data. Reflection state of the micromirror is set in accordance with a value of the one bit mirror drive data. Next, the second highest bit of K-bit image data is picked up and written to the micromirror device. The pulse width modulation of the image data is performed by reducing write cycle of the mirror drive data to half. When the value of the image data is large, total time of the valid reflection state becomes long so that great amount of the spot light is projected. When the value of the image data is small, small amount of the spot light is projected.
The area-type digital micromirror device may be employed in the electrophotography-type printer as well. During rotation of photosensitive drum, N micromirror arrays are driven at the same time in accordance with the image data of first to Nth lines to expose the N lines of the image data on the photosensitive drum. In order to reduce the unsharp of line due to the rotation of the photosensitive drum, exposure time is set at 10% of the movement time of one line.
On an outer peripheral surface of the photosensitive drum, electrostatic latent image is formed by the digital micromirror device. The electrostatic latent image is developed and converted to toner image. The toner image is transferred to a plain paper. On each line of the electrostatic latent image, line multiple exposure is performed by N micromirror arrays. As to the line multiple exposure, the number of exposures is one of zero to N and determined on the basis of the image data. The line multiple exposure has an advantage that harmful effect due to nonuniformity of light or scatter of operational speed of the micromirror is reduced.
A method for recording a high gradation image is disclosed in Japanese Patent Laid-open Publication No. 7-131648 which is Japanese counterpart of U.S. patent application No. 038,391 filed on Mar. 29, 1993. In the method, for some of N micromirror arrays, time of the valid reflection state thereof is changed during each exposure. And for the rest of the N micromirror arrays, the time of the valid reflection state is fixed during each exposure.
It is possible to realize a color printer for recording a color image in photosensitive material by utilizing the above-described micromirror device. In this case, three kinds of light sources, namely light sources of red, green and blue, are necessary. The photosensitive material is intermittently advanced one line by one line and the three colors are recorded in order while the photosensitive material is stopped. In such a case, it is difficult to locate the photosensitive material every one line so that it is impossible to advance the photosensitive material at high speed. As a consequence of that, there arises a problem in that it takes time to print the color image.